1. Field of the Invention
The present invention relates to wavelength division multiplexed (WDM) optical systems, and more specifically, it relates to optical systems, in which independent channels on different optical wavelengths are simultaneously broadcast to many nodes over a star coupler.
2. Description of Related Art
A key barrier to higher performance levels in massively parallel processors (MPPs) is the communication limits that exist among the individual processors, and between the processors and main memory. Such communication limits include delays in message transmission that could be reduced, e.g., by increasing the transmission bandwidth. The time delay for transmission of a large message reduces in proportion with the transmission bandwidth of the communication link transporting the data. Additional time delays between initial message transmission and reception stem from the use of information packets that are relayed many times, e.g., in a bucket-brigade fashion from node-to-node within a communication fabric. At each such node, the packet address header is read to route each message packet appropriately to its intended destination. If this occurs more than once, unnecessary latency in the delivery of the message packet is added and can stall processors waiting for the data. Performance suffers when the processors are starved of needed data. The processors cannot continue until all the required packets are received. The efficiency of parallel systems falls off as systems are scaled up to include more processors because of the above-mentioned latency and bandwidth limitations. As the system size, measured in number of processors, grows, each processor spends more time waiting for data. Such problems have been encountered by the Cray Research Torus program with three-dimensional interwoven rings, the Intel Paragon mesh program with two-dimensional rings without wrap-around, and the Convex Exemplar program where the symmetric multiprocessor (SMP) groups are on parallel rings.
Multiprocessing is of great current interest for both general high performance computing applications, massively parallel processing, and integrated sensor/processor systems. Increases in system node count, computing power per node, and/or sensor-generated data rate increase the communication required to maintain a balanced system that fully utilizes available computing power and sensor data. Traditional electronic solutions are not keeping pace with advances in processor performance and sensor complexity, and have increasing difficulty providing sufficient communication bandwidth. The trend towards shared memory (away from message passing) in multiprocessors places additional stress on inter-processor communications due to the short messages and rapid memory access associated with cache-to-cache coherence traffic.
The difficulty of providing sufficient communication resources between processor and memory elements in parallel, multiprocessor systems has led to many proposals to employ optical interconnects for improved bandwidth and latency. These proposals are driven by communication requirements anticipated from significant increases in computing power per node (1 GFLOPS per CPU near term) and system node count, and the recognition that traditional electronic interconnects will have increasing difficulty in meeting these requirements. Enhanced interconnects are required to provide sufficiently rapid access to remote, distributed memory so that available computing power is fully utilized for applications requiring tightly coupled multiprocessing. Cache-coherent, shared memory operation places additional stress on inter-element communications due to the short messages and rapid memory access associated with cache-to-cache coherence traffic.[6] In addition, rapid remote access can significantly improve memory requirements, and thus system cost, for certain scientific codes (e.g.: in which complex, underlying physics is represented by look-up tables), because large quantities of read-only data need not be replicated locally.
It is well known that the latency in a communications fabric can be reduced by increasing the xe2x80x9cdegreexe2x80x9d of the network, which is the number of nodes (processors, memories or sensors) which can be accessed for communication by a given node without the necessity of intervening routing logic. A high network degree minimizes the number of times a packet header is processed en route to its destination, and thus minimizes the latency. This has led to several proposals to use fiber optic interconnects for multiprocessors, because the fiber optic media enables a broadcast architecture involving many nodesxe2x80x94that is, a high network degree. The typical architecture involves a broadcast architecture (embodied as a star coupler) and wavelength-selectable node transmitters. The multiple optical wavelengths in the network enable multiple, simultaneous communication transmissions involving different sets of source/destination node pairs.
The use of wavelength-division-multiplexed (WDM) optical systems (FIG. 1), in which independent channels on different optical wavelengths are simultaneously broadcast to a large number (e.g.: hundreds) of nodes over a star coupler, is an attractive proposal for multiprocessor interconnects, offering the potential for wide-bandwidth, single-hop communications among all nodes. Each wavelength provides an independent, concurrent logical bus channel. With sufficient system wavelengths, it provides a non-blocking crossbar interconnect (output contention only), and can lead to a knockout switch (no output contention) given sufficient receiver resources. While scaling of such systems is ultimately limited by the optical power budget and bandwidth limitations of the optical transceiver technology, use of bridged WDM star couplers as multi-ported ported routers or spanning busses can enable scaling to higher node count. The large degree/fanout of such routers/busses is attractive for minimizing system diameter and global communication latency.
In previously proposed, conventional architectures of this type, in which a single pair of optical fibers is used to transport information to and from each node, there exists a fundamental tradeoff between the number of nodes on the star coupler (the network degree) and the transmission bandwidth. An information source must provide sufficient optical power to transmit to many destinations simultaneously because optical receivers will not produce error-free outputs unless they receive strong optical signals. The required optical signal strength increases with increasing bandwidth. When there are a lot of destinations, and the node degree increases, a larger amount of power is required. However, optical power cannot be increased indeterminately because of other system constraints, including the cost of high power laser transmitters, maximum device power limits, and the desire to operate with xe2x80x9ceye-safexe2x80x9d laser powers in the network. These constraints on maximum transmission power will force the system to operate with lower transmission bandwidth when the number of nodes on a star coupler is increased. This is an undesirable option, which occurs in a variety of multiwavelength optical architectures based on broadcast-and-select type architectures, including those using n-to-n broadcast, n-to-n star couplers, or n-to-1 combining in the optical domain suffer from the power inefficiencies of 1/n, where n is the number of nodes on the network. The hardware design is complicated as more wavelengths are required to be emitted from each node in a system.
Examples of the type of architecture described above are presented by
Charles Husbands in U. S. Pat. No., 5,446,572;
E. Arthurs et al., Electron. Lett. 24, 119 (1988);
K. Ghose, xe2x80x9cPerformance Potentials Of An Optical Fiber Bus Using Wavelength Division Multiplexingxe2x80x9d, Proc. SPIE 1849, 172-183 (1993);
M. Goodman et al, xe2x80x9cThe LAMBDANET Multiwavelength Networkxe2x80x9d, IEEE J. Sel. Areas in Communications vol. 8, no 6, pp 995-1004 (1990); and
H. Obara and Y. Hamazumi, in xe2x80x9cStar Coupler Based Wavelength Division Multiplexer Switch Employing Tunable Devices With Reduced Tunability Rangexe2x80x9d, Electronics Letters, Jun. 18, 1992; Vol. 28, No. 13, pp. 1268-1270.
Charles Husbands describes in U. S. Pat. No. 5,446,572, a broadcast architecture in which the optical power is broadcast from each transmitter into a common channel connected to every receiver in the system. Such combining reduces the power available to each connection by 1/n, where n is the number of wavelength division multiplexers being combined. So a lot of optical power is required from each transmitter to begin with, and the transmitter power must be increased with each transmitter/receiver node added to a system. High levels of optical power reduce reliability, increase power consumption, and can prevent the system from being xe2x80x9ceye safexe2x80x9d for maintenance personnel. But reducing the overall power even as the number of nodes increase forces lower bit rates, because the receiver sensitivity requirements for error-free operation at high bit rate will be exceeded.
Sasayama et al., describe in U. S. Pat. No. 5,506,712, a time-slotted, synchronized wavelength division multiplexing approach to connect each of m node inputs to some number of outputs. It employs a wavelength router instead of an optical star coupler to overcome the optical power splitting associated with a star coupler. Such frequency routers typically introduce an optical attenuation which increases with the number of inputs and outputs on the router, and therefore impose the same tradeoff of network degree versus transmission bandwidth as discussed above.
Sotom describes in U.S. Pat. No. 5,485,297, an optical switch that uses tunable wavelength division multiplexing sources, and optical switch matrices plus star couplers to route wavelength division multiplexing transmissions to a particular destination. The purpose of the switches is to minimize the size of the star coupler to improve optical power utilization and minimize the number of system wavelengths required by routing messages on the same wavelength to different star couplers. The disadvantage of this approach is the need for a centralized control that analyzes the traffic pattern for the inputs and then sets all the switches to make sure two signals on the same wavelength never go to the same star. This kind of centralized control is slow, complex, and costly.
Sharony et al. describes in U.S. Pat. No. 5,495,356, a time-slotted approach that requires global synchronization. Optical space switches, e.g., photonic switches in FIG. 4 of the patent, or wavelength switching are used for wavelength selective switching. Centralized control is needed to operate such switches, which is complex, slow, and costly. Sharony et al. also uses 1:n splitting which is power inefficient, and has limited switch tuning times.
M. Kavehrad and M. Tabiani describe in, xe2x80x9cSelective Broadcast Optical Passive Star Coupler Design For Dense Wavelength Division Multiplexer Networksxe2x80x9d, IEEE Photonics Letters, vol. 3, no. 5, May 1991, pp. 487-489, reducing the splitting loss power inefficiency by selectively broadcasting through an optical star coupler to limit broadcasts to only a few nodes. The proposed device appears complicated to build, and attempts to tradeoff splitting losses against the number of system wavelengths used. cl SUMMARY OF THE INVENTION
It is an object of the present invention to provide wavelength-division-multiplexed (WDM) optical systems, in which independent channels on different optical wavelengths are simultaneously broadcast to many (over 100) nodes over a star coupler with a large transmission bandwidth.
The invention offers wide-bandwidth, single-hop communications among a large number of nodes. Each wavelength provides an independent, concurrent logical bus channel. With sufficient system wavelengths, it provides a non-blocking crossbar interconnect (output contention only), and can lead to a knockout switch (no output contention) given sufficient receiver resources (e.g., LAMBDANET). While scaling of such systems is ultimately limited by optical power budget and transceiver bandwidth, use of bridged WDM star couplers as multi-ported routers or spanning busses enables scaling to higher node count. The large degree of such routers is attractive to reduce system diameter and global communication latency. An embodiment of the invention includes a basic WDM star-coupled system. The invention addresses the concern that the interconnect hardware provides robust, scalable performance at the level at or beyond 100 sustained GFLOPS and a few hundred nodes.
An embodiment of a transmitter module design of the present invention provides ≈1 nsec wavelength selection, broadcast capability, and large output power using a single module containing two optoelectronic chips. The first chip contains an array of laser diodes, each emitting at a different wavelength. The second chip contains two arrays of semiconductor optical amplifiers (SOAs) interconnected by a passive star coupler. The lasers emit continuously and may be collimated and focused by micro-optics. The transmitter wavelength is selected in the optical domain by using a first SOA array as an electro-optic switch. A wavelength select circuit controls this first SOA array. The second SOA array is controlled by an electronic driver array and provides modulators to impress word-wide electronic data onto the word-wide spatial channels (a multimode fiber array) realized via broadcast over the star coupler.